This invention relates to amphoteric polymer particles having select acid functional groups. In particular, the polymer particles are useful for preparing composite particles, which comprise inorganic particles, each having amphoteric polymer particles in contact therewith. The invention further relates to an aqueous composition comprising the composite particle. The present invention is suitable for preparing dried coatings.
Opacifying pigments provide whiteness, and opacity or “hiding”, to opacifying coatings, such as paints. These pigments are present in all coatings that are designed to provide an opaque coating on an undersurface or substrate surface to which the coating is applied. Opacifying pigments are absent from those coatings that are designed to be clear or transparent.
It is desirable that opacifying coatings have a high opacifying capacity to completely conceal the undersurface, while using a minimal application of the coating. It is highly desirable that complete covering of the undersurface is attained with a single application of the coating or paint, having the minimum possible thickness.
Opacifying coating manufacturers have long sought to formulate opacifying coatings having the desired opacity by maximizing the level of hiding for a defined level of opacifying pigment to minimize the amount of opacifying pigment utilized.
The opacifying capacity or hiding power of an opacifying coating is a measure of the coating's ability to conceal a surface to which the coating is applied. Opacifying capacity is a function of the spacing between the particles of opacifying pigment in the dried applied coating, and is maximized when the light scattering capability of the opacifying pigment is maximized. Maximum light scattering efficiency occurs when the opacifying pigment particles have a certain diameter and spacing, so that the light scattering capability of each particle does not interfere with the light scattering capability of its neighboring particles. This condition may occur in coatings containing sufficiently low levels of opacifying pigment such that the individual opacifying pigment particles are isolated from each other. Coatings containing such low levels of opacifying pigment, however, do not provide sufficient whiteness and hiding at typical dried coating thicknesses. Achieving the desired levels of hiding and whiteness typically requires higher levels of opacifying pigment. At these higher levels, a statistical distribution of opacifying pigment particles occurs, which results in at least some of the opacifying pigment particles being in such close proximity to each other so as to produce a loss of light scattering efficiency due to crowding of the opacifying pigment particles.
Increased hiding efficiency is obtained by reducing the crowding of the opacifying pigment particles and minimizing the formation of clusters of opacifying pigment particles. One method uses polymer particles containing select chemical groups that promote adsorption to the opacifying pigment particle.
For example, U.S. Pat. No. 7,179,531 discloses an aqueous dispersion of composite particles that contain multiphase polymer particles adsorbed to a titanium dioxide opacifying pigment particle. The selected multistage polymer particles have dihydrogen phosphate functional groups, which promote adsorption of the selected polymeric latex particles onto the surface of the titanium dioxide particles.
Although these composite particles provide improved hiding, there is still a need to provide aqueous compositions suitable for preparing dried coatings having increased hiding. Other desired properties for the dried coatings include increased gloss and whiteness.